The present invention relates to rotating electrical machines that are used as electric power generators in Integrated Power Systems (IPS) where different loads are powered from the same generator. In particular, this invention relates to synchronous machines, both the wound field and the permanent magnet type, used to generate power in systems in which a plurality of voltages are required and constant service load voltage regulation is required. Of particular interest is such an IPS in naval vessels having propulsion system loads and service loads.
A typical generator arrangement in an IPS consists of a wound field synchronous machine or multiple machines that generate Alternating Current (AC) voltage power. This power is distributed to the electric propulsion loads as propulsion power (the voltage of which sometimes is referred to herein as propulsion voltage) and to the service loads as service power (the voltage of which is sometimes referred to herein as service voltage). Because of the typically large power requirements of electric propulsion loads relative to those of the service loads, the propulsion voltage is typically much higher than the service voltage. When this is the case, step-down transformers are typically utilized to lower the generator voltage to the desired service voltage. In addition, transformers are also sometimes included at the front-ends of propulsion motor drives to reduce current harmonics and resultant voltage distortion on the main power distribution bus. As a result, the size, weight, and cost of the IPS are greatly affected and increase with each added component.
For many marine applications, extreme power density is a fundamental requirement, i.e., marine applications require more compact systems with greater power output for a given system size. A typical implementation of a port/starboard integrated power system is illustrated in FIG. 1.
As illustrated in FIG. 1, a prime mover 1a provides rotational power to a synchronous generator 2a. Power produced by the generator 2a is typically at a medium voltage, i.e., 5-15 kV, and can be alternating at a standard rate of 50 or 60 Hertz (Hz), or as high as 400 Hz.
The generated power is typically distributed through switchgear 3a and power cabling 4 in a ring configuration to allow connection of other devices such as other generators 2b, propulsion motor 5 and service loads 6a and 6b. Propulsion transformers 7a and 7b are often included to decrease the effect of the propulsion system harmonic currents on the system. The propulsion transformers 7a and 7b feed power at a transformed voltage to propulsion motor drives 8a and 8b, respectively, which in turn are connected to the propulsion motor 5. Motor 5 in turn is operatively coupled to a screw 10. Service load transformers 9a and 9b are typically utilized to reduce the voltage of the generated power to desired service load voltages which can be AC or Direct Current (DC) (if rectified), as typically is done on present Naval combatant platforms. The service load transformers 9 in turn provide power to the service loads 6a and 6b, respectively, at the reduced service load voltages.
Submarine integrated power system architectures have been studied several times. The studies drew conclusions regarding the optimal power distribution system architecture, based on the data and assumptions available at the time of study, foreseen or expected. These data and assumptions included:                a. 1,200-1,700 VDC power semiconductor devices as the only devices capable of providing submarine-level power quality;        b. In early studies, commercial drivers for the development of higher voltage semiconductors were not expected;        c. Single-level basic power structures (H bridges); and        d. Conventional electric fault protection devices and conventional coordination systems and methods.        
More recent studies have concluded that AC systems are superior to DC systems for the next generation submarine propulsion plant development, superior being measured by a combination of risk, acoustic performance viability and power density. The conclusion was most heavily influenced by the power quality and rating available in the cascaded H-bridge topology (so called “Robicon drive”) and the viability of conventional medium voltage vacuum breakers to operate at higher frequencies (120-400 Hz). This conclusion was also supported by the relative ease with which the power system could be scaled to larger platforms.
Subsequent to a majority of these studies, there have been advancements in commercial industry.                Power semiconductors (both IGBTs and IGCTs) are available and are in relatively widespread use at up to 6 kV ratings. 3 kV devices are widely employed in light rail applications, and higher voltage devices are used in rail and in industrial drives, such as mining, metals and petrochemicals.        Multi-level (e.g. neutral point clamped) power structures are widely employed in industrial applications, and high performance military applications, such as the drive for the Permanent Magnet Motor Subsystem (PMMS) successfully tested in the Navy's Philadelphia LBTS facility.        
A majority of naval applications rely on power conversion for ship service power, rather than conventional 60 Hz power provided by electric machines. Systems sourced by power conversion equipment have fundamentally different fault current characteristics than systems sourced by rotating electric machines.
U.S. Pat. Nos. 6,504,261 and 6,333,622, incorporated herein in their entireties for all purposes, state that auxiliary windings in synchronous generator/motor machines have been proposed in Naval ship propulsion systems. In particular, the main generator windings would provide power to electric motors coupled to the propeller shaft. The auxiliary windings would provide power to the shipboard power distribution system for lights, motors and other ship functions.
Both of these patents discuss the use of a synchronous generator having main and auxiliary power windings, where the main power windings are coupled to a so-called balanced power system and the auxiliary power windings are selectively coupled to a variable frequency drive system (a so-called static start drive) or to an auxiliary power system (the so-called balance of auxiliary power system). The variable frequency drive system causes the generator to function as a motor and turn a drive shaft to start a gas turbine. Once the turbine is up and running, the generator functions as a generator. The auxiliary power system comprises systems that can be powered by the lower voltage power signal output by the generator when in power generation mode. Switching circuits are used to connect and disconnect the auxiliary windings of the generator between the variable frequency drive system and the auxiliary power system.